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LM5143DESIGN-CALC: body diode reverse loss calculation

Part Number: LM5143DESIGN-CALC
Other Parts Discussed in Thread: LM5143-Q1, LM5145

Hi team,

here is two confusing items of LM5143q design:

1.there is a descripton as following in the datasheet what seems not to make sense, actually it suppose that there is no body diode reverse loss of high side mos, right?

Page 37: Typically two-thirds of the net loss attributed to body diode reverse recovery 

if above description is right or not?

2. there is a section that is called external schottky diode in the design calculator excel

if this schottky diode is placed parallel with low side mos for better reverse performance to achieve higher efficiency? 

If so, how to choose this diode, if the current range need to be high as the low side mos as the diode will run in freewheeling stage for a while when low side mos off?

Thanks.

Best regards

Mia Ma

  • Hi Mia,

    Thanks for the question related to the LM5143-Q1 quickstart calculator.

    Q1: The body diode reverse recovery creates a current spike, which increases switching loss mostly in the high-side FET. Some loss is also attributes the the low-side FET. We assume a 2 : 1 distribution between the FETs in the calculator.

    Q2: If an antiparallel Schottky diode is connected in parallel with the low-side, we assume the Qrr of the body diode is no longer applicable (i.e. all the current transfers to the Schottky during the deadtime). Typically, we do not install this diode as the deadtime is so short for this controller (in practice, the diode will likely not conduct as the parasitic connection inductance prevents full current transfer/commutation).

    Regards,

    Tim

  • Hi Hegarty,

    thanks for your reply. 

    Q1: what in my mind is that high side mos doesn't have a frewheeling stage like the low side mos do; so the body diode reverse is only shown in low side mos. If any misunderstanding here?how to understand the current spike increase switching loss in the high side FET?

    Q2: our design is aimed to deliver 6V/20A output single phase, the reverse loss is taking a siginicant role here. So it's still not to recommend use a schottky diode. Or if use one, how to choose the current limit

    Thanks

    Best regards

    From Mia

  • Hi Mia,

    Q1: Both FETs have integral body diodes, but only the low-side FET body diode is relevant in a buck regulator. The leading-edge current spike is higher given the reverse recoery of the low-side FET body diode, hence the increased switching losses.

    Q2: Can you check the Qrr of the low-side FET. Also, take a look at the LM5145 5V/20A EVM - there should be no need for an antiparallel diode given the short deadtime, but you can include one if you feel it may provide a benefit.

    Regards,

    Tim

  • Hi Hegarty,

    Got it and thanks.

    Regards,

    Mia